Abstract

The National Ignition Facility (NIF) provides a unique opportunity to study implosion physics with nuclear yield. The use of polar direct drive (PDD) [A. M. Cok, R. S. Craxton, and P. W. McKenty, Phys. Plasmas 15, 082705 (2008)] provides a simple platform for the experimental studies without expensive optics upgrades to NIF. To determine the optimum PDD laser pointing geometry on NIF and provide a baseline for validating inertial confinement fusion codes against experiments for symmetric and asymmetric implosions, computer simulations using the 3D radiation-hydrodynamics code hydra [M. M. Marinak, R. E. Tipton, O. L. Landen, T. J. Murphy, P. Amendt, S. W. Haan, S. P. Hatchett, C. J. Keane, R. McEachern, and R. Wallace, Phys. Plasmas 3, 2070 (1996)] were preformed. The upper hemisphere of a DT-filled CH capsule was imploded by 96 NIF beams in a PDD configuration. Asymmetries in both polar and equatorial directions around the capsule were observed, with the former dominating the latter. Analysis of the simulation results indicates that the lack of symmetry in the initial power density profile (during the first 200 ps of the implosion) is a primary cause of late-time asymmetry in the implosion as well as decreased yield. By adjusting the laser pointings, the symmetry and total neutron yield were improved. Simulations with dropped quads (four of the NIF laser system’s 192 beamlines) without repointing worsen the overall symmetry by a factor of 10 (with respect to rms radial variation around the capsule) and reduce neutron yield by a factor of 2. Both of these degraded implosion characteristics are restored by azimuthal repointing of the remaining quads.

Received 22 August 2011Accepted 15 November 2011Published online 11 January 2012

Acknowledgments:

This research was supported by US DOE/NNSA, performed at LANL, operated by LANS LLC under Contract No. DE-AC52-06NA25396. The authors are extremely grateful to Marty Marinak and the rest of the HYDRA team for making their code available to us to perform this work. We also would like to express our gratitude to Larry Suter and Ines Heinz for facilitating computational access to HYDRA.